1. Field of the Invention
The present invention relates to an image capturing apparatus for increasing the precision of a captured image by compensating for image blur caused by hand-shake.
2. Description of the Related Art
In current cameras, all of the important operations required for shooting a picture, such as metering and focusing, are automated. Therefore, even persons inexperienced at operating cameras can easily shoot a picture. In addition, recently, cameras with a system for improving image blur caused by hand-shake have become available, thus eliminating most of the factors that can cause a photographer to make a mistake when shooting a picture.
An optical image stabilization technology, which is one of the available image stabilization systems for improving image blur caused by hand-shake, is described briefly. In general, the hand-shake of a camera at image capturing time is a vibration having a frequency of 1 to 10 Hz. To improve blurring of a captured image caused by hand-shake at exposure time, the vibration of the camera caused by hand-shake is detected to enable the displacement of a correction lens in a plane perpendicular to the optical axis in accordance with the detection result.
Accordingly, to carry out a precise image stabilization operation, the following two operations are required to be carried out: the accurate detection of the vibration of a camera and the accurate correction of the vibration of the optical axis on the basis of the detected vibration. To detect hand-shake, the acceleration, angular acceleration, angular velocity, and angular displacement of a camera are detected using a component such as a laser gyro, and a vibration detection unit for carrying out an arithmetic operation on the detection result is mounted in the image capturing apparatus. Thereafter, an optical correction apparatus is driven to deflect the shooting axis on the basis of the detected information about hand-shake provided by the vibration detection unit. Thus, blurring of the captured image can be improved.
According to Japanese Patent No. 3110797, image capturing device of a video camera accumulates electric charge (performs an exposure) in 1/1000 of a second for an odd field and in 1/60 of a second for an even field of an image signal. The area in the odd field that is saturated to black is corrected using an image signal in an even field in accordance with an exposure period to generate a corrected odd field signal. The area in the even field that is saturated to white is corrected using an image signal in an odd field in accordance with an exposure period to generate a corrected even field signal. Thus, the dynamic range of the image signal is virtually increased.
Image vibration can be improved using the time-sharing exposure method disclosed in Japanese Patent No. 3110797. For example, a determined exposure period is divided into a plurality of shorter ones each of which is sufficiently short to improve image blur caused by hand-shake, and an image capturing operation is repeated the number of times necessary for this division. The plurality of images captured by the plurality of capturing operations are combined while improving the shift among the images. As a result, the captured image (combined image) corresponding to the determined exposure period can be obtained.
To further improve the image blur caused by hand-shake, in addition to the above-described image stabilization system, a camera body may employ the above-described technique. In this case, an image is repeatedly captured using a shutter speed that is sufficiently high to avoid image blurring.
However, the shutter speed required to restrain image blurring within tolerable range (image stabilization limit) may be different depending on the performance of an optical image stabilization system in a mounted interchangeable lens. For example, one interchangeable lens having a focal length of 300 mm has an image stabilization limit of 1/60-second shutter speed. Another lens having a focal length of 300 mm has an image stabilization limit of 1/30-second shutter speed. In this case, the appropriate shutter speed for divided image capturing and the number of images that are captured are different for the two lenses.
More specifically, as the exposure period becomes longer, the noise of an image decreases. Accordingly, each exposure period for divided image capturing can be increased with increasing image stabilization performance of an interchangeable lens. Thus, a superior image with reduced noise can be obtained. Additionally, a longer time-sharing exposure period reduces the number of time-sharing exposures, which in turn reduces the number of shutter operations and the number of image data transfers. As a result, the image capturing time for obtaining a combined image can be reduced.